1. Field of the Invention
The present invention relates to a system for controlling an electric vehicle motor, which system optimally discharges a smoothing capacitor, connected to the input terminal of an inverter, in accordance with the condition of the electric vehicle.
2. Description of the Related Art
As disclosed in Laid Open No. Hei 4-165901 and Laid Open No. Hei 5-184016, conventional systems for controlling an electric vehicle motor are known to be arranged so that high DC power supply voltage (ranging, for example, from 150 to 300 V) generated by a plurality of batteries (e.g., lead-acid batteries) connected in series with each other is converted into AC power supply voltage (for example, AC voltage with three phases U, V, and W), using a DC-AC converter such as an inverter, following which conversion the resulting AC power supply voltage is supplied to a vehicle drive motor to drive a vehicle.
In conventional systems for controlling an electric vehicle motor, a large-capacity smoothing capacitor for compensating for variations in DC power supply voltage is inserted into the input terminals of inverters because DC power supply voltage on the input side of the inverters varies with AC power supply voltage output therefrom due to load variations involved in start-up, shutdown, and rapid acceleration and deceleration of the motor.
As described above, conventional systems for controlling an electric vehicle motor are arranged so that a smoothing capacitor inserted into the input terminals of inverters is used to supply stable DC power supply voltage to a vehicle drive motor even if the load applied to the motor varies.
Being arranged as described above, however, conventional systems for controlling an electric vehicle motor have a problem of the charge accumulated in their smoothing capacitors being left even after an operation switch operated by the driver, for example, an ignition switch is turned off, thereby stopping the electric vehicle, and thus the capacitors must wait to be spontaneously discharged when the systems undergo maintenance.
When a vehicle is at rest with the ignition switch thereof OFF, the charge in the large-capacity smoothing capacitor is no longer increased, and thus the voltage across the smoothing capacitor can be lowered by discharging the smoothing capacitor, using a discharging circuit, for example, a resistor.
When an electric vehicle is bein towed or drawn with the ignition switch thereof OFF, or when the ignition switch is turned off while the electric vehicle is running, regenerative effect accompanying the rotation of the vehicle drive motor may cause voltage on the input side through the inverter.
Since reducing the capacity of a discharging circuit for discharging the smoothing capacitor, which is charged as a result of continuous voltage occurrence at the input terminal of the inverter due to the regenerative effect lasting while the vehicle is towed or drawn, leads to heat generation from the discharging circuit, it is undesirable to discharge the smoothing capacitor when the vehicle is being towed or drawn.
On the other hand, increasing the capacity of the discharging circuit for the purpose of preventing heat generation causes an increase in cost.